1. Field of the Invention
The invention relates to a tightening device for the gripping jaws of a plate stretcher, comprising, per pair of gripping jaws, one hydraulically slidable push rod on which there are swivel-mounted two link rods which are each hinged to one gripping jaw, and these gripping jaws move synchronously forwards for closing and backwards for opening in two guides disposed in V formation relative to each other.
2. Description of the Prior Art
Hydraulic tightening devices are used for pressing the gripping jaws against the plate to be gripped, in which a hydraulic cylinder is connected to a push rod on which there are hinged two rods which themselves are connected by joint bolts to two opposing gripping jaws guided in guides of V formation. This is necessary in order to ensure synchronous sliding of the two opposing gripping jaws. This tightening device operates under the following conditions:
1. If the plate becomes torn, the gripping jaws experience a reaction due to the torn plate, which imparts a temporary acceleration to the gripping jaws of more than 100 g. Because of the inertia of the mass of the tightening device, very high forces arise in the connection bolts, which can lead to damage to the hinges.
2. It happens that the toothed gripping surfaces of the gripping jaws, and particularly the lower gripping jaw, become partly covered by shaved-off pieces of the stretched plates, thus resulting in a considerable reduction in the friction coefficient, i.e. the plate is gripped initially or mainly only by one gripping jaw, whereas the other gripping jaw remains behind or falls back. Thus because of the different gripping jaw movement, opposing high forces arise in the rods, which frequently lead to damage of the tightening device.
In addition, wrinkle formation or tearing occur in the plate to be stretched, i.e. the plate either cannot be used or must be newly straightened, heat-treated and stretched.
In order to prevent damage in these two cases, spring elements have been inserted into the joints between the push rods and the two link rods. By this means, in case 1, the mass of the accelerated rod is indeed reduced, but not eliminated. Experience shows that even with this solution, the joint bolts must be very frequently replaced due to deformation. The high spring force required because of the necessary pressing force and the large spring deflection required in case 2 considerably limit the effectiveness of this measure, both in case 1 and in case 2. This measure is also made more difficult by the fact that such devices are accessible only with great difficulty, and long idle periods must be assumed in the case of trouble.